1. Field of the Invention
The present invention relates generally to a frequency shift keying modulator (FSK modulator), and more particularly, to a frequency shift keying modulator having sigma-delta modulated phase rotator.
2. Description of Related Art
In recent years, along with the rapid development of wireless communication technology, transmission data rate of wireless transmitters is becoming higher. The wireless transmitters generally comprise phase-locked loop (PLL) for precisely locking the transmission frequency thereof.
Referring to FIG. 1, a block diagram of a closed-loop direct-modulation wireless transmitter 10 comprising a phase-locked loop 11 is shown. The wireless transmitter 10 comprises a phase-frequency detector (PFD)/charge pump (CP) 102, a loop filter (LF) 104 coupled to the phase-frequency detector (PFD)/charge pump (CP) 102, a voltage-controlled oscillator (VCO) 106 coupled to the loop filter 104, a frequency divider 108 coupled between the voltage-controlled oscillator 106 and the phase-frequency detector (PFD)/charge pump (CP) 102, a driver 110 coupled to the voltage-controlled oscillator 106, and an antenna 112 coupled to the driver 110, wherein the phase-frequency detector/charge pump 102, the loop filter 104, the voltage-controlled oscillator 106 and the frequency divider 108 constitutes a phase-locked loop 11.
The frequency divider 108 divides the frequency of the voltage-controlled signal fVCO outputted from the voltage-controlled oscillator 106 by a frequency division ratio related to the transmission data; the phase-frequency detector/charge pump 102 compares the result outputted from the frequency divider 108 with a reference signal fREF and generates a voltage-raising or voltage-dropping signal according to the difference therebetween such that the voltage-controlled signal fVCO generated by the voltage-controlled oscillator 106 can be locked and is equal to the reference signal fREF multiplied by the frequency division ratio. Equivalently, the frequency of the voltage-controlled signal fVOC is the frequency division ratio times as high as the frequency of the reference signal fREF. In other words, by adjusting the frequency division ratio, the frequency of the voltage-controller signal fVCO can be changed.
The closed-loop direct-modulation wireless transmitter 10 has advantages of low power consumption and stable center frequency, but as bandwidth of data is limited by the phase-locked loop 11, the phase-locked loop 11 can only be applied in narrow-band transmission signal.
Referring to FIG. 2, a block diagram of an opened-loop direct-modulation wireless transmitter 20 having another kind of phase-locked loop 21 is shown. Different from the wireless transmitter 10 of FIG. 1, besides the phase-frequency detector (PFD)/charge pump (CP) 102, the loop filter (LF) 104, the voltage-controlled oscillator (VCO) 106, the frequency divider 108, the driver 110 and the antenna 112, the wireless transmitter 20 further comprises a switch 202 coupled between the loop filter 104 and the voltage-controlled oscillator 106. In addition, the transmission data is inputted to the voltage-controlled oscillator 106 instead of into the frequency divider 108. In the wireless transmitter 20, the phase-frequency detector/charge pump 102, the loop filter 104, the voltage-controlled oscillator 106, the frequency divider 108 and the switch 202 constitute a phase-locked loop 21.
The disposing of the switch 202 in the wireless transmitter 20 helps the wireless transmitter 20 having the phase-locked loop 21 overcome the drawback of the wireless transmitter 10 having the phase-locked loop 11 that the wireless transmitter can only be used in narrow band transmission signal. But the wireless transmitter 20 has a drawback of center frequency drift.